A killer is loose in tranquil Richmond-on-Thames, and it's up to Detective Inspector Sean Connor to catch him before it's too late.
A series of shocking and particularly gruesome murders lead Sean and his team into a labyrinthine investigation, in which all roads lead towards a thirty-year-old unsolved murder. The victims, all apparently unconnected to each other, are being dispatched by the use of a singularly unique poison, previously associated with the notorious Borgia family.
As the murders begin to multiply at an alarming rate, Connor finds clues hard to come by, and every lead takes him down yet another blind alley. The killer seems to be one step ahead of the police at every turn. Together with his assistant, Sergeant Lucy Clay, they must piece together the shreds of evidence that will lead them to the mysterious Chocolate Woman, and in turn to the brain behind the horrific murders that soon come to be known as The Purple Death.